Page 41 - Origin and Prediction of Abnormal Formation Pressures

24 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III

certain depth [Ph, lb/ft 2 = (depth, D, ft) x (specific weight, gw, lb/ft3)].
Lithostatic (or geostatic)gradient, GI: the total pressure exerted by the overburden
(rocks plus interstitial fluids) per unit of depth.
Lithostatic (or geostatic)pressure, Pl: the pressure exerted by the total weight of the
rocks and the interstitial fluids at a particular depth.
Skeletal loading: that portion of the lithostatic pressure borne by the framework of
solids (rock) of the porous media. This pressure is often referred to as the effective
pressure, Pe or O-e.

COMPACTION PROCESS

Several mechanisms responsible for generating abnormal pressures (Tables 2-1 and
2-2) are related to the changes in rock pore volume. This is particularly true in the
case of relatively young basins that are buried from 1.0 to 2.0 km (Swarbrick and
Osborne, 1998) and that had been rapidly deposited, or in those older basins with
thick sections of fine-grained sediments. The increase in pore pressure results from the
fact that the interstitial solution cannot escape quickly enough from the shrinking rock
structure.

Hydrostatic pressure

The hydrostatic pressure, Ph is defined as the pressure exerted at the bottom of a
vertical column of water (Fig. 2-1) extending from the surface:

Ph -- Yw D = 0.433D (2-1)
and where ),'w is the specific weight of water, and D is the length of the column of water.
The pressure gradient of pure water is equal to 0.433 psi/ft.
The specific weight of a fluid is a function of the quantity of dissolved solids
(salinity), water temperature and volume of dissolved gases. Fig. 2-2 demonstrates
the effect of total dissolved solids on the specific weight of the fluid. As the salinity
increases (content of dissolved solids), the specific weight also increases. Fig. 2-3
illustrates the effect of dissolved gas, pressure and temperature on the compressibility
and pressure gradient of water.

Formation or interstitial fluid pressure

Formation (interstitial pore fluid) pressure deviating from the hydrostatic pressure,
Ph, at any depth, D, is identified as abnormal formation pressure. Several examples of
geopressure gradients are shown in Fig. 2-4. As discussed by Watts (1948), isolated
abnormally high formation pressures (AHFP) may be found in the Ventura Field,
California, as a result of thrusting. High pressures in formations associated with salt
domes along the Gulf Coast of Texas and Louisiana are often attributed to faulting
and diastrophism accompanying intrusion of the salt domes. A typical formation water
gradient in this area is about 0.465 psi/ft (0.074 kg cm -2 m-l), which corresponds to